This invention relates to a vehicle brake system, and more particularly, to a vehicle brake system which includes a secondary (emergency) brake function and a hydraulic valve which closes off a failed brake circuit and permits the other circuit to function normally.
The John Deere 8000T tractor has a hydraulic braking system which includes a velocity fuse spool valve which is located downstream of the brake valves. The velocity fuse valve includes a spool which is normally held in an open flow position by centering springs. The centering springs, in conjunction with the pilot areas on the spool define a differential pressure which must exist between the two brake circuits before a lock-out of one circuit occurs. The lockout occurs due to low pressure in the failed circuit. Once the low pressure in the failed circuit is low enough to produce the predetermined pressure differential, the pressure differential will shuttle the spool and close off the failed circuit. After the spool has shuttled, the other circuit is allowed to function normally. However, typically such a valve will shuttle at a pressure differential which is low enough such that normal, intermittent, short duration variations in brake pressure may cause undesired shuttling of the velocity fuse valve spool. It would be desirable to control or limit the sensitivity of such a velocity fuse valve and prevent undesired shuttling of the velocity fuse valve spool in such situations.